Problem: $ C = \left[\begin{array}{rrr}2 & 1 & 1 \\ 2 & 0 & 2 \\ 0 & 1 & 1\end{array}\right]$ What is $ C^{-1}$ ?
Explanation: $ = \left[\begin{array}{rrr}\frac{1}{2} & 0 & -\frac{1}{2} \\ \frac{1}{2} & -\frac{1}{2} & \frac{1}{2} \\ -\frac{1}{2} & \frac{1}{2} & \frac{1}{2}\end{array}\right]$